This application is the national phase under 35 U.S.C. xc2xa7371 of PCT International Application No. PCT/JP99/00680 which has an International filing date of Feb. 17, 1999, which designated the United States of America.
This invention relates to an NOx reducing method for decreasing the amount of NOx generated by the combustion of a gas generating agent for air bag in an inflator of an air bag system installed in automobiles, airplanes, etc, for protecting human bodies.
This invention relates further to an inflator adopting the above NOx reducing method and an inflator system using the above NOx reducing method and the inflator.
Sodium azide is well known at present as a combustion component of a gas generating agent used in air bag systems. The gas generating agent using sodium azide has a high decomposition temperature of 400xc2x0 C. or above and is excellent in thermal stability. This agent has no problem of its combustion characteristics, in particular, and has therefore gained a wide practical application. However, it is known that sodium azide might result in environmental pollution when it is discarded in large quantities because, for example, sodium azide forms explosive compounds upon reacting with heavy metals, and its peroral toxicity LD50 of a mouse and a rat is 27 mg/kg.
To solve these problems, novel compounds that may substitute for sodium azide have been examined. For example, JP-B 6-57629 discloses a gas generating agent containing a transition metal complex of tetrazole or triazole. JP-A 5-254977 discloses a gas generating agent containing triaminoguanidine nitrate. JP-A 6-239683 discloses a gas generating agent containing carbohydrazide. JP-A 7-61885 discloses a gas generating agent that contains cellulose acetate, potassium perchlorate and an nitrogen-containing nonmetallic compound. U.S. Pat. No. 5,125,684 discloses a gas generating agent containing 15 to 30% of a cellulose base binder such as nitrocellulose, and an energetic material. JP-A 4-265292 discloses a gas generating composition comprising the combination of tetrazole and triazole derivatives, an oxidizing agent and a slag-forming agent.
A gas generating composition composed of a nitrogen-containing organic compound has, in general, defects such that the heat content is large, a combustion temperature is high, a linear burning rate is small and a trace amount of poisonous gas generates, in comparison with a gas generating composition of an inorganic azide compound when it burns with an oxidizing agent to generate oxygen gas in the stoichiometric amount, that is, in so much an amount as to burn non-oxidized elements such as carbon, hydrogen and others contained in the molecule of the nitrogen-containing organic compound.
Among the gases generated by the combustion of the gas generating agent, it is CO and NOx that render the particular problem. These gases are generated substantially always when organic compounds are burn. When the amount of the organic compound is greater than the theoretical amount of complete oxidation, the trace CO concentration in the generated gas increases, and when the amount of the organic compound is equal to, or smaller than, the theoretical amount of complete oxidation, the trace NOx concentration in the generated gas increases, although the absolute numerical values vary with the kinds of the organic compound and the oxidizing agent used for the gas generating agent. Both of them never reach zero in the combustion of the gas generating agent, and the range within which the optimal balance between them can be maintained must be searched. From the aspect of protection of users, it is demanded that the concentrations of the generated CO and NOx gases should be further reduced, however, no effective method of reducing these has yet to be in practical use. Particularly because the NOx concentration does not much decrease even when the mixing ratio between the organic compound and the oxidizing agent is changed, it is extremely difficult to reduce the NOx concentration.
One of the known NOx reducing methods is a denitration technology in a large-scale process using a boiler and a burner for burning heavy oil, kerosene, coal, propane gas, and so forth. A typical example of this denitration technology is the selective reducing method using NH3 for removing NOx by the following reaction:
NO+NH3+xc2xcO2xe2x86x92N2+3/2H2O
One may attempt to utilize this selective reducing method for the inflator. However, it is not possible to use NH3which is a gas (boiling point: xe2x88x9233.4xc2x0 C.) for a reducing agent. The method using urea in place of the NH3 gas is known, but this method cannot be applied to practical application because decomposition and sublimation take place in the high temperature test required for the inflator.
WO98/06682 discloses an NOx reducing method using the reducing material selected from the group consisting of ammonium salts, typified by (NH4)2CO3, (NH4)2SO4, NH4Cl, H2NCO2NH4 and NH4F, ammonium hydroxide, amine compounds, amide compounds, typified by H2NCONH2, and imide compounds typified by cyanuric acid (HNCO)3. However, these reducing materials involve various problems caused from the factors such that they have low thermal stability and that they are likely to undergo decomposition with the passage of time.
In the thermal stability test at 105xc2x0 C., for example, the weight loss ratio of both of (NH4)2CO3 and H2NCO2NH4 after 18 hours was 100%, whereas the weight loss ratio of H2NCONH2 after 408 hours in the same test was 2.11%. When the reducing materials are decomposed and their weight decreases in this way, a sufficient reduction effect cannot be exhibited, and the rise of the internal pressure due to the generation of the decomposition gas may break the seal of the vessel. Furthermore, ammonia and amino radicals, etc. as the decomposition gas of the reducing materials have high reactivity, so that the decomposition of the gas generating agent itself is induced and therefore, service life of the inflator is shortened. Incidentally, though cyanuric acid is hardly decomposed by heat, its reducing power is low as is obvious from its structure. Therefore, its NOx reducing effect is small.
The reducing materials disclosed in WO98/06682 generate harmful gases to the human body upon decomposition. For example, H2S and SOx are generated from (NH4)2SO4; HCl and Cl2 are generated from NH4Cl; and H2F and F2 are generated from NH4F. Leak of these harmful gases outside the inflator system cannot be neglected when the influences on the human body are taken into consideration, and also lowers safety of the system as a whole.
It is an object of the present invention to provide a NOx reducing method, in an inflator of an air bag system, such that the amount of the NOx generated by the combustion of a gas generating agent for the air bag can be reduced, and therefore, safety can be further improved in view of the protection of users.
In the inflator for the air bag, the method of the present invention reduces NOx, generated by the combustion of the gas generating agent, using a reducing material or its decomposition products, and reduces the amount of these NOx.
The inflator generally includes a ignition means, the gas generating agent and a coolant/filter.
Preferably, the reducing material is placed in an ignition means accommodation chamber, or in a combustion chamber storing the gas generating agent, or at a position in the proximity of a gas outlet from the ignition means accommodation chamber inside the combustion chamber that stores the gas generating agent, or at a coolant/filter portion.
Preferred examples of the reducing materials include amide compounds, imide compounds, amine compounds, guanidine derivatives, tetrazole derivatives, hydrazine derivatives, triazine derivatives, hydroxylamine salts, sodium salts, NH4OH, ammonium salts, cyanates, dicyanamide salt, ammine complexes and CDH complexes. More concretely, they include azodicarbonamide, dicyandiamide, tetrazole compounds or their salts, bitetrazole compounds or their salts, and triazole compounds or their salts. More preferably, they include the mixture of the tetrazole compounds or their salts with copper oxides. Among the tetrazole compounds, 5-aminotetrazole is most preferred.
The reducing materials includes the form of at least one member selected from the group consisting of amide compounds, guanidine derivatives, tetrazole derivatives, hydrazine derivatives, triazine derivatives, hydroxylamine salts, sodium salts, ammonium salts, ammine complexes, cyanates and dicyanamide salts.
The reducing materials may be molded articles.
The present invention provides further an inflator for an air bag that includes a gas generating agent and a reducing material. The inflator comprises a ignition means, the gas generating agent and a coolant/filter, and further includes preferable factor such that the gas generating agent and the reducing material are placed apart from each other by a partition plate to be ruptured by the gas pressure generated at the time of combustion.
The present invention further provides an air bag system comprising the inflator, an impact sensor, control means for inputting a detective signal and outputting an operation signal to the ignition means of the inflator and an air bag. The present invention further provides a method of reducing NOx, generated by the combustion of the gas generating agent, using a reducing material and reducing their amount adopted bin the air bag system.
To find a suitable method of reducing NOx, the inventors of the present invention has made studies specifically aiming at the selective reducing method using urea, and has found that the objects of the invention described above can be accomplished by utilizing the reaction between radicals and NOx on the basis of the concept that the NOx reducing mechanism by urea results from the reaction between radicals such as NH2 radicals, NCO radicals, CN radicals, alkyl radicals, and so forth, with NOx, and a part of NOx is changed to the N2 gas. The present invention has thus been completed.
In other words, the present invention provides the NOx reducing method characterized in that NOx generated by the combustion of the gas generating agent for the air bag is reduced by the reducing material placed inside the inflator and their amount is reduced.
The present invention further provides an inflator comprising a gas generating agent and a reducing material.
The present invention provides further an inflator system using the NOx reducing method or the inflator described above.
When the NOx reducing method according to the present invention is employed, it becomes possible to reduce the NOx generated by the combustion of the gas generating agent for the air bag by the reducing material placed inside the inflator, and to thus reduce the NOx amount. The reducing material used for the NOx reducing method according to the present invention has higher thermal stability than that of the reducing materials of the prior art which are different from the reducing material of the present invention. Moreover, the reducing materials of the present invention hardly generate poisonous gases.
Therefore, when the NOx reducing method, and the inflator using this method, according to the present invention are employed, the possibility of adverse influences on the health of users can be lowered, and reliability as well as safety of the inflator system can be further enhanced.